The present invention relates generally to laser recording systems in which a digitally modulated laser beam is deflected by a rotating mirror onto a flat recording surface and more particularly to a novel arrangement for generating timing signals for compensating for changes in the angular velocity of the swept beam.
Recording systems which employ a laser for transferring digital information from a computer to a light sensitive recording surface are well known to those skilled in the art. Generally, such systems rely on a laser beam which is deflected by a rotating mirror, or other type of scanning device, and modulated in accordance with digital data trains representative of positive and negative areas or various optical densities or grey levels to be reproduced on the recording surface. In normal operation, the recording surface is moved either continuously or in a stepping manner as it is being swept by the digitally modulated laser beam.
In order to accommodate the use of a rotating light beam, the recording surface is often maintained in a concave configuration over the area that is scanned by the light beam. An example of this type of laser recording system may be found in U.S. Pat. No. 4,054,928 to J. C. Butler etc. wherein the recording medium is supported on a curved platen. Examples of laser recording systems wherein the recording surface is maintained in a concave configuration but wherein the laser beam is modulated with analog signals rather than digital signals may be found in U.S. Pat. No. 3,154,621 to W. R. Hohnson, U.S. Pat. No. 3,816,652 to G. B. Barnett and U.S. Pat. No. 3,874,621 to L. R. Blair etc.
In many instances, it is either not possible or not desirable to maintain the recording surface in a concave configuration over the area that is scanned by the rotating light beam. For example, the laser recording system may be one in which the recording medium is a photoconductive drum whose cylindrical shape is rigid and cannot be altered. As the rotating light beam is swept across the drum, the surface that is exposed to the light beam is substantially straight or flat. An example of a laser recording system in which a digitally modulated laser beam is swept across the surface of a photoconductive drum may be found in U.S. Pat. No. 4,124,286 to S. Barasch.
As is known, a train of digital signals is linear; that is, the series of pulses are evenly spaced from one another. However, if a rotating light beam is used to transfer digital signals to a recording surface and the recording surface is flat at the area exposed to the light beam, the light spots that are produced on the recording surface corresponding to the digital signals will be unevenly spaced because of the changing angular velocity of the beam as it sweeps across the surface.
For many applications, the distortions resulting from the uneven placement of the light spots are not acceptable.
One prior art technique for compensating for the changing angular velocity of a rotating light beam as it is swept across a flat surface and thereby avoiding the distortions is to incorporate a special lens system which is designed to make the necessary corrections. Some of the problems with this approach are that the lens system that is needed for this purpose is fairly complex, somewhat cumbersome and somewhat costly to manufacture. An example of a laser recorder employing a compensating lens system of this type may be found in the November 1979, issue of the Bell System Technical Journal.
Another prior art technique for compensating for the changing angular velocity of a rotating light beam as it is swept across a flat surface is to generate timing signals which are used to control the flow of the digital data used to modulate the laser beam so that the light spots corresponding to the digital signals evenly spaced on the flat surface. Although a number of different arrangements have been proposed or actually reduced to practice for generating the timing signals, none of the arrangements have proven to be entirely adequate or satisfactory.
For example, in U.S. Pat. No. 3,389,403 to J. M. Cottingham there is disclosed a data plotter in which timing signals are generated for this purpose by deflecting an auxiliary light beam across a hollow bar having a series of evenly spaced slots. Light entering the bar through the slots is sensed by a photodiode located at one end of the bar. In the November 1979, issue of Bell System Technical Journal, there is disclosed an arrangement for generating timing signals by deflecting an auxiliary light beam across a flat code plate having a series of evenly spaced light transmissive portions. Light passed through the transmissive portions of code plate is collected by a lens located behind the code plate and directed into a suitably positioned phototube.